docs: def is_mapped_luks_device(dev):
      """
      Determine if dev is a mapped LUKS device
      :param: dev: A full path to a block device to be checked
      :returns: boolean: indicates whether a device is mapped
      """
      _, dirs, _ = next(os.walk(
          '/sys/class/block/{}/holders/'
          .format(os.path.basename(os.path.realpath(dev))))
      )
      is_held = len(dirs) > 0
      return is_held and is_luks_device(dev)